An RFID system that uses a UHF band has a longer communication range than an electromagnetic induction (or “induction field”) RFID system, which uses an HF band, and, for that reason, has been used mainly in the field of logistics. Accordingly, an RFID tag is in some cases attached to a metal object (conductive object) for use.
When attached to a metal object or installed near a metal object, a conventional RFID tag that uses a dipole antenna has a problem in that the antenna ceases to work or the communication range diminishes greatly under the influence of the metal object. Realizing an RFID tag that is capable of reducing performance degradation when attached to a metal object is therefore demanded.
In view of this demand, there has been proposed an RFID tag that can be installed in an object irrespective of whether the object is conductive or non-conductive by mounting an RFID IC chip in a slot provided in a radiating conductor of a microstrip antenna (see Patent Literature 1, for example).
The operating frequency band of a microstrip antenna that is used in an RFID tag described in Patent Literature 1 is a narrowband because no measure is provided to maintain the impedance characteristics of the microstrip antenna in a broader band.
At present, UHF-band RFID systems use different frequencies in different regions: 952 to 954 MHz in Japan, 902 to 928 MHz in the United States, and 865 to 868 MHz in Europe.